The present invention relates to the field of location determining, and, more particularly, to proximity detection of a person or object carrying a tag.
Proximity detection devices are used in a wide variety of applications for determining the relative nearness or separation of an object or person in relation to another object or person. Approaches of recent public interest focus on child monitoring when both the child and the guardian are within close proximity, for example less than 15 fifteen feet apart. For many other applications, this separation distance may be undesirably close.
Low frequency magnetic field generation is described in U.S. Pat. No. 5,477,210 to Belcher. The system described uses low power magnetic field generation providing sharper proximity threshold performance than high frequency approaches, with separate sequentially generated and encoded transmissions to accommodate the random orientations inherent in both portable units. The small size of the generating unit makes this approach only practical for short distances where the size of the magnetic field generation circuitry is limited. Longer range performance, as would be appropriate for child monitoring from a fixed residence necessitates 100 to 150 foot range.
Radio frequency based proximity detection systems may require the installation of a buried antenna to define a desired boundary or perimeter. Other radio frequency based proximity detection schemes based upon radiated RF signals from a central or base station may have difficulty in establishing a sharp perimeter. In addition, RF signals may be severely attenuated when passing through conventional building materials and other objects. Accordingly, the field strength may vary greatly from the desired geometry.
In view of the foregoing background, it is therefore an object of the invention to provide a proximity detection system which is readily installed, and which has a sharply defined perimeter.
These and other objects, features and advantages in accordance with the present invention are provided by a proximity detection system comprising a magnetic field generator for generating a rotating magnetic field having a decreasing intensity over an increasing separation distance, and a magnetic field detector being relatively movable and generating a crossing indication. This crossing indication is based upon an intensity threshold in the rotating magnetic field being crossed as a threshold separation distance from the magnetic field generator is crossed. The system may also include a transmitter for transmitting a signal relating to the crossing indication from the magnetic field detector.
The magnetic field generator may generate a substantially constant amplitude rotating magnetic field vector, and the magnetic field detector may comprise a plurality of orthogonal detection coils. The system may include a portable housing containing the magnetic field detector and the transmitter thereby defining an object or person tag. The magnetic field generator is typically considered as the base station, and need not necessarily be portable. The rotating magnetic field provides a relatively sharp cut-off threshold separation distance defining a perimeter for proximity detection. Of course, the proximity detection could be used to determine when the tag passes in either direction through the perimeter, although typically the system may be used to detect when the tag moves outside the perimeter.
The system may further include a receiver remote from the transmitter for receiving signals from the transmitter. The receiver may be located adjacent the magnetic field generator. The transmitter and receiver may operate at radio frequencies, for example.
In one embodiment, the transmitter may transmit a series of signals based upon an intensity of the rotating magnetic field being greater than the intensity threshold indicative that the magnetic field detector is proximate the magnetic field generator. The transmitter may stop transmitting the series of signals based upon an intensity of the rotating magnetic field being less than the threshold intensity. The system may also include an alert indicator connected to the receiver for generating an alert indication based upon failure to continue to receive the series of signals from the transmitter. Accordingly, if the transmitter fails or stops transmitting because the tag is outside the defined perimeter, an alert indication may be given.
In another embodiment, the transmitter transmits a signal based upon an intensity of the rotating magnetic field being less than the intensity threshold indicative that the magnetic field detector is no longer proximate the magnetic field generator. In this embodiment, an alert indicator may also be connected to the receiver for generating an alert indication responsive to the transmitter transmitting the signal.
The magnetic field generator may include a modulator for modulating the rotating magnetic field. The magnetic field detector may also include a demodulator which is selectively responsive to the modulated rotating magnetic field. In other words, the base station and tag can be coded by modulation so that the tag is only responsive to the correct base station and would not be responsive to a nearby base station. For example, the modulator may modulate the rotating magnetic field to have at least one of a predetermined rotation direction and a predetermined rotation speed.
The magnetic field generator may include a drive circuit and at least one pair of orthogonal coils connected thereto. Moreover, the drive circuit may have an adjustable output to set a desired separation distance corresponding to the intensity threshold. Accordingly, the threshold separation distance or perimeter can be readily set by adjusting the output of the magnetic field generator.
The drive circuit may also have a predetermined frequency and the at least one pair of coils have a predetermined size. This causes the rotating magnetic field to be generated while substantially no electric field is generated.
In accordance with another advantageous aspect of the invention, the magnetic field generator may comprise a spread spectrum modulator. The magnetic field detector will therefore include a spread spectrum demodulator. In addition, the magnetic field detector may use at least one of received signal strength and correlated signal energy from the spread spectrum modulated rotating magnetic field.
A method aspect of the invention is also for proximity detection. The method may include generating a rotating magnetic field having a decreasing intensity over an increasing separation distance from a base station, and providing a tag for an object or person that is relatively movable with respect to the base station. The tag may preferably comprise a magnetic field detector for generating a crossing indication based upon an intensity threshold in the rotating magnetic field being crossed as a threshold separation distance from the base station is crossed, and a transmitter for transmitting a signal relating to the crossing indication.